PROLOG This manual page is part of the POSIX Programmer
s Manual. The Linux implementation of this interface may
differ (consult the corresponding Linux manual page for
details of Linux behavior), or the interface may not be
implemented on Linux.

NAME expr - evaluate arguments as an expression

SYNOPSIS expr operand

DESCRIPTION The expr utility shall evaluate an
expression and write the result to standard output.

OPTIONS None.

OPERANDS The single expression evaluated by expr shall
be formed from the operands, as described in the EXTENDED
DESCRIPTION section. The appli- cation shall ensure that
each of the expression operator symbols:

( ) | & = > >= < <= != + - * / % :

and the symbols integer and string in the table are
provided as sepa- rate arguments to expr.

STDIN Not used.

INPUT FILES None.

ENVIRONMENT VARIABLES The following environment
variables shall affect the execution of expr:

LANG Provide a default value for the
internationalization variables that are unset or null. (See
the Base Definitions volume of IEEE Std 1003.1-2001, Section
8.2, Internationalization Vari- ables for the precedence of
internationalization variables used to determine the values
of locale categories.)

LC_ALL If set to a non-empty string value, override the
values of all the other internationalization variables.

LC_COLLATE

Determine the locale for the behavior of ranges,
equivalence classes, and multi-character collating elements
within regular expressions and by the string comparison
operators.

LC_CTYPE Determine the locale for the interpretation of
sequences of bytes of text data as characters (for example,
single-byte as opposed to multi-byte characters in
arguments) and the behavior of character classes within
regular expressions.

LC_MESSAGES Determine the locale that should be used to
affect the format and contents of diagnostic messages
written to standard error.

NLSPATH Determine the location of message catalogs for
the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS Default.

STDOUT The expr utility shall evaluate the expression
and write the result, followed by a <newline>, to
standard output.

STDERR The standard error shall be used only for
diagnostic messages.

OUTPUT FILES None.

EXTENDED DESCRIPTION The formation of the expression to
be evaluated is shown in the follow- ing table. The symbols
expr, expr1, and expr2 represent expressions formed from
integer and string symbols and the expression operator sym-
bols (all separate arguments) by recursive application of
the con- structs described in the table. The expressions are
listed in order of increasing precedence, with
equal-precedence operators grouped between horizontal lines.
All of the operators shall be left-associative.

Expression Description expr1 | expr2 Returns the
evaluation of expr1 if it is neither null nor zero;
otherwise, returns the evaluation of expr2 if it is not
null; otherwise, zero. expr1 & expr2 Returns the
evaluation of expr1 if nei- ther expression evaluates to
null or zero; otherwise, returns zero. Returns the result of
a decimal integer comparison if both arguments are inte-
gers; otherwise, returns the result of a string comparison
using the locale-spe- cific collation sequence. The result
of each comparison is 1 if the specified relationship is
true, or 0 if the rela- tionship is false. expr1 = expr2
Equal. expr1 > expr2 Greater than. expr1 >= expr2
Greater than or equal. expr1 < expr2 Less than. expr1
<= expr2 Less than or equal. expr1 != expr2 Not equal.
expr1 + expr2 Addition of decimal integer-valued argu-
ments. expr1 - expr2 Subtraction of decimal integer-valued
arguments. expr1 * expr2 Multiplication of decimal
integer-valued arguments. expr1 / expr2 Integer division of
decimal integer-val- ued arguments, producing an integer
result. expr1 % expr2 Remainder of integer division of
decimal integer-valued arguments. expr1 : expr2 Matching
expression; see below. ( expr ) Grouping symbols. Any
expression can be placed within parentheses. Parentheses can
be nested to a depth of {EXPR_NEST_MAX}.

integer An argument consisting only of an (optional)
unary minus followed by dig- its. string A string argument;
see below.

Matching Expression The : matching operator shall
compare the string resulting from the evaluation of expr1
with the regular expression pattern resulting from the
evaluation of expr2. Regular expression syntax shall be that
defined in the Base Definitions volume of IEEE Std
1003.1-2001, Section 9.3, Basic Regular Expressions, except
that all patterns are anchored to the beginning of the
string (that is, only sequences starting at the first
character of a string are matched by the regular expression)
and, therefore, it is unspecified whether ^ is a special
character in that context. Usually, the matching operator
shall return a string repre- senting the number of
characters matched (0 on failure). Alterna- tively, if the
pattern contains at least one regular expression subex-
pression "(..)]", the string corresponding to
1" shall be returned.

String Operand A string argument is an argument that
cannot be identified as an inte- ger argument or as one of
the expression operator symbols shown in the OPERANDS
section.

The use of string arguments length, substr, index, or
match produces unspecified results.

EXIT STATUS The following exit values shall be
returned:

0 The expression evaluates to neither null nor zero.

1 The expression evaluates to null or zero.

2 Invalid expression.

>2 An error occurred.

CONSEQUENCES OF ERRORS Default.

The following sections are informative.

APPLICATION USAGE After argument processing by the
shell, expr is not required to be able to tell the
difference between an operator and an operand except by the
value. If "$a" is, the command:

expr $a ==

looks like:

expr = = =

as the arguments are passed to expr (and they all may be
taken as the =operator). The following works reliably:

expr X$a = X=

Also note that this volume of IEEE Std 1003.1-2001
permits implementa- tions to extend utilities. The expr
utility permits the integer argu- ments to be preceded with
a unary minus. This means that an integer argument could
look like an option. Therefore, the conforming applica- tion
must employ the "--" construct of Guideline 10 of
the Base Defini- tions volume of IEEE Std 1003.1-2001,
Section 12.2, Utility Syntax Guidelines to protect its
operands if there is any chance the first operand might be a
negative integer (or any string with a leading minus).

EXAMPLES The expr utility has a rather difficult
syntax:

* Many of the operators are also shell control operators
or reserved words, so they have to be escaped on the command
line.

* Each part of the expression is composed of separate
arguments, so liberal usage of <blank>s is required.
For example:

In many cases, the arithmetic and string features
provided as part of the shell command language are easier to
use than their equivalents in expr. Newly written scripts
should avoid expr in favor of the new fea- tures within the
shell; see Parameters and Variables and Arithmetic Expansion
.

The following command:

a=$(expr $a + 1)

adds 1 to the variable a.

The following command, for "$a" equal to
either /usr/abc/file or just file:

expr $a :.*(.| $a

returns the last segment of a pathname (that is, file).
Applications should avoid the character / used alone as an
argument; expr may interpret it as the division
operator.

The following command:

expr "//$a" :.*(.)

is a better representation of the previous example. The
addition of the "//" characters eliminates any
ambiguity about the division operator and simplifies the
whole expression. Also note that pathnames may con- tain
characters contained in the IFS variable and should be
quoted to avoid having "$a" expand into multiple
arguments.

The following command:

expr "$VAR" :.*

returns the number of characters in VAR.

RATIONALE In an early proposal, EREs were used in the
matching expression syntax. This was changed to BREs to
avoid breaking historical applications.

The use of a leading circumflex in the BRE is
unspecified because many historical implementations have
treated it as a special character, despite their system
documentation. For example:

expr foo : ^foo expr ^foo : ^foo

return 3 and 0, respectively, on those systems; their
documentation would imply the reverse. Thus, the anchoring
condition is left unspeci- fied to avoid breaking historical
scripts relying on this undocumented feature.

FUTURE DIRECTIONS None.

SEE ALSO Parameters and Variables, Arithmetic
Expansion

COPYRIGHT Portions of this text are reprinted and
reproduced in electronic form from IEEE Std 1003.1, 2003
Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the
Institute of Electrical and Electronics Engineers, Inc and
The Open Group. In the event of any discrepancy between this
version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .